Development of gradient-wetting Janus wood membrane with high-efficiency fog collection and oil-water separation

Abstract

Conventional preparation of Janus membranes with asymmetric wettability focuses on the preparation of extreme hydrophilic side and hydrophobic side, but suffers from pore clogging, low liquid transfer and collection efficiency and short service life. In this study, Janus wood (JW) with a continuous wettability gradient was created throughunidirectional vacuum impregnation of a hydrophobic solvent after the delignification of wood. Digital image correlation (DIC) and layer-by-layer contact angle assessment were utilized to examine the thickness-dependent wettability gradient of the JW membrane. The JW membrane completed the modification of the cell walls without blocking the cell lumens. The presence of wettability gradient in the JW membrane effectively reduced the capillary force of liquid penetration, resulting in decreased adhesion and stagnation of liquid on the membrane surface, and increased flow rate and transport efficiency. Based on the continuous wettability change of the JW membrane, the JW membrane was endowed with the ability to act as a “unidirectional valve” to enable continuous and rapid directional transport of liquids. The mechanical properties of the JW membrane prepared by the unidirectional introduction method were significantly improved, rendering them highly stable and environmentally durable. The fog collection efficiency of the JW membrane was maintained at 97.7% over 10 repeated use cycles. For light oil/water and heavy oil/water mixtures, JW membrane maintained an efficiency of 99.6% or higher. Unlike conventional surface modification methods, the unidirectional introduction method proved practical and effective for modifying the JW membrane in the thickness direction. Furthermore, the prepared JW membrane was sustainable, inexpensive, durable, and environmentally friendly. This study provides a wide range of potential applications in fields such as droplet manipulation, multi-liquid separation, industrial wastewater purification, and microfluidics.